Systems have been designed and installed to deter or repel aquatic animals, especially juvenile fish, from underwater areas, for example, the water inlets of hydroelectric and steam electric power plants. Considerable research has been conducted in support of attempts to develop non-physical barriers which are effective against one or more selected species of aquatic animals. Known systems have employed such devices as underwater strobe lights, sound generators and electric fields. Exemplary systems are shown, for example, in Menezes U.S. Pat. No. 4,992,468, Suomala U.S. Pat. No. 4,932,007, and Loeffelman U.S. Pat. No. 4,955,005.
Good avoidance response is reported to low frequency water particle oscillating motion by downstream migrating Atlantic salmon smolt (Salmo salar) in F. R. Knudsen et at, Journal of Fish Biology (1994) 45, 227-233. An earlier paper by Knudsen et al on the same topic notes the lack of commercially available means for producing underwater sound or particle motion at frequencies below about 30 Hz. Knudsen et al, Journal of Fish Biology (1992) 40, 523-534. A piston device for generating such low frequency motion is shown in FIG. 3 of the more recent Knudsen et al paper, having an upper end exposed to the atmosphere and a lower end submerged below the surface of the water. The piston is mounted at the underwater end of the cylinder, attached to a motor and crank for reciprocating motion with a 4 cm. peak-to-peak travel range. Water is drawn into the cylinder on the back stroke of the piston within the cylinder and is expelled out of the cylinder on the forward stroke of the piston. Results are reported in the Knudsen et al paper for the low frequency (10 Hz.) water particle motion generated by such piston device.
Prior known fish deterrent systems have had insufficient efficacy against all or selected fish species. A system effective in repelling one species may prove ineffective in repelling other target species. In other cases, a system initially effective against one or more target species has grown less effective over time due to habituation. Lack of adequate component durability also has been a problem in certain cases involving such complex componentry. Furthermore, known systems employing electromechanical devices, such as underwater loud speakers, have undesirably high equipment costs and/or installation and operating costs. Operating costs often are a significant factor in determining the commercial suitability of such systems.
Accordingly, it is an object of the present invention to provide a system for repelling selected aquatic animals from an underwater zone, having good efficacy against selected species as well as commercially acceptable durability and costs, including operating costs. Additional objects will be apparent from the following disclosure of the invention and detailed description of certain preferred embodiments.